If you think Iron Maiden didn't make a ton of money on this tour, you're crazy. It used to be that bands toured to sell albums. That's often still the case, as there is still plenty of money to be made in CD sales and downloads, notwithstanding all the illegal downloading. These days, bands make lots and lots of money touring. You don't sell out entire soccer stadiums without making out very well financially. Iron Maiden certainly could have made more money than they did -- they could have charged $150 per ticket instead of $60 -- but I don't buy the notion that touring is a money-losing, or even a break-even, proposition. All that said, I agree that merchandise is a big, big part of the band's income. So are catalog sales. They probably get sizable royalty checks every quarter for albums they recorded 20-25 years ago, as people are still buying those CDs, and all expenses attributed to those titles were recouped a long, long time ago. Finally, as is the case with almost everyone who wisely invests his wealth early in life, most of their net worth is probably attributed to capital gains on their investments rather than new income from touring, record sales or merchandising.
EDIT: To SMX's point about artist advances, that's still the case, even for big-time artists. The difference, though, is that the advances are much, much bigger. Note the $100+ million record/touring deals signed by artists like U2, Madonna, Michael Jackson and Jay-Z. I used to represent PolyGram Records when it still existed, and believe me, U2 never got a single royalty from albums like Pop or Zooropa, because they signed a huge deal (after Achtung Baby, I believe) that never got recouped and ended up being a huge disappointment for the record company. Still, people sign established acts to very lucrative extensions, and if they are negotiated aggressively by the artist's lawyer, the advance may be more than the total royalties, even on successful albums. Allen Grubman, one of the most powerful music attorneys in the world, has been known to say (I'm paraphrasing, but this is pretty close): "If my clients ever see a penny in royalties, I haven't done my job." Meaning, the advance he negotiated was too small.